Invert emulsion fluids. i.e. emulsions in which the non-oleaginous fluid is the dispersed phase and the oleaginous fluid is the continuous phase, are employed in drilling processes for the development of oil or gas sources, as well as, in geothermal drilling, water drilling, geoscientific drilling and mine drilling. Specifically, the invert emulsion fluids are conventionally utilized for such purposes as providing stability to the drilled hole, forming a thin filter cake, lubricating the drilling bore and downhole, and penetrating salt beds without sloughing or enlargement of the drilled hole.
Invert emulsion fluids generally consist of a three-component system, namely an oleaginous liquid such as oil which serves as a continuous phase, a non-oleaginous liquid such as water which serves as a discontinuous phase, and an emulsifying agent. The emulsifying agent serves to lower the interfacial tension of the liquids so that the non-oleaginous liquid may form a stable dispersion of fine droplets in the oleaginous liquid. A full description of such invert emulsions may be found in Composition and Properties of Drilling and Completion Fluids, 5th Edition, H. C. H. Darley, George R. Gray, Gulf Publishing Company, 1988. pp. 328-332, the contents of which are hereby incorporated by reference.
Usually, lime or other alkaline materials are added to conventional invert emulsion fluids to maintain a reserve alkalinity when the invert emulsion fluid is to be used in drilling. See, for example, API Bulletin RP 13B-2, 1990. p. 22 which describes a standard test for determining excess lime in drilling mud. See also, for example, U.S. Pat. No. 5,254,531 which employs lime along with an ester oil, a fatty acid, and an amine and EP 271943 which employs lime along with oil, water, and an ethoxylated amine. In some areas acidic gases such as CO.sub.2 or H.sub.2 S are encountered during drilling. The acidic gases often weaken conventional invert emulsion fluids to the point of failure and render the invert emulsion fluids unstable.
Another aspect of invert emulsions that is typically encountered during drilling is the need to increase the density of the drilling fluid. Conventionally this is done by the addition of solid weighting agents or by the use of brines of salts that form basic or neutral aqueous solutions. The use of brines of salts that form acidic aqueous solutions, for example a transition metal halide solution, is difficult because of the addition of the acidic brine would tend to make the invert emulsion fluids unstable. Further, due to the alkaline reserve noted above, the transition metal ion would precipitate out due to the formation of hydroxide complexes. However the use of acidic brine solutions is desired due to their higher density imparted by the metal cation.
Thus, it would be desirable if an invert emulsion could be invented which did not fail or weaken when subjected to acidic gases such as CO.sub.2 or H.sub.2 S. Likewise, it would be desirable to weight an oil based drilling fluid with high density acidic brines which may be made with acidic metal salts. In this manner, the invert emulsion could be employed as a stable drilling or completion fluid.